An exhaust gas filtering system having a diesel particulate filter (which is referred to as DPF) disposed in an exhaust pipe of a diesel engine is well known. The DPF captures particulates (which is referred to as PM) contained in exhaust gas from the diesel engine. The DPF is periodically regenerated by combusting the accumulated PM according to accumulated PM quantity which is estimated from a differential pressure between upstream and downstream of the DPF.
JP-2004-068804A (US-2003/0230060A1) shows a problem of the exhaust gas filtering system that temperature of the DPF is over-rising. A steep combustion of the accumulated DPF causes a steep temperature rising of the DPF, so that it may cause damages of the DPF and a deterioration of catalyst supported by the DPF. This over-temperature-rising of the DPF is tend to arise especially when temperature of the exhaust gas flowing into the DPF is relatively high due to high-load driving of the engine, or when the flow rate of the exhaust gas passing through the DPF is steeply decreased in a situation that the temperature of the DPF is high by a regenerating operation thereof. As shown in FIG. 2A, it is because that heat radiation quantity HRAQ radiating from DPF into the exhaust gas is steeply decreased to steeply increase the temperature of the DPF.
In order to restrict the temperature rising, even if an operation is conducted to reduce the heat quantity HREQ transferring into the DPF from the exhaust gas, for example, the exhaust gas temperature is decreased or unburned HC is stopped to be supplied to the DPF, it is hard to restrict the temperature rising of whole of the DPF because the downstream portion of the DPF receives heat from the upstream portion of the DPF via the exhaust gas flowing therethrough. It is necessary to increase the exhaust gas quantity flowing through the DPF so that the HRAQ into the exhaust gas is increased to restrict the temperature rising of the downstream portion of the DPF. JP-2004-068804A shows the system in which the exhaust gas quantity flowing into the DPF is increased when the over-temperature-rising may occur.
However, when the exhaust gas flow rate is increased too much during regeneration thereof, the temperature of the upstream portion of the DPF is considerably reduced. Thus, fuel is wasted to increase the temperature of the DPF again, so that fuel economy is deteriorated.